Coil tubing is primarily used to perform various down hole operations in oil and gas wells. The depth of the well can be many thousands of feet which makes the continuous coil tubing reel very heavy and in some situations impossible to move in one piece. In offshore rigs, the weight of the coil tubing reel is limited by crane capability and other logistical issues related to the harsh working environment, which requires the coil tubing to be transported in two or three reels. Conventional methods of joining coil tubing requires a certified welder to weld two ends of coil tubing together without significantly de-rating the fatigue limit of the coil tubing, which is in the range of 30-40% for a manual butt weld. However, certified welders are very expensive and not always readily available. The equipment needed to insure a high integrity weld is also expensive and not always readily available. Furthermore, the weather conditions can make the welding operation a significant challenge.
There are several coil tubing connectors on the market which have attempted to address some of these issues. A dimple connector of the type shown in U.S. Pat. No. 6,474,701 is one example of such a connector. It uses a dimpling method to join two ends of the coil tubing to a central connector. The center of the connector is formed with radial slots filled with elastomeric pieces. The dimple connector has an acceptable fatigue life and exhibits a good tensile strength, however, the elastomeric material is not suitable in all fluid environments. Furthermore, this design requires a hydraulic dimpling tool on location.
A simple roll-on type connector has also been proposed. However, such connectors do not have a good torque rating and hence are not practical for joining two ends or sections of coil tubing. Other connectors, such as slip connectors and splined connectors, are not spoolable and therefore are also not practical for joining spoolable coil tubing.
Therefore, there is a need in the coil tubing industry for a connector which has approximately the strength of the base coil tubing, can be spooled easily on a reel with sufficient fatigue life for multiple spooling/unspooling operations, requires minimal equipment and time to install, and has sufficient torque imparting characteristics for typical coil tubing operations.